In medium-long distance optical communication systems, the speed has been increasing and the capacity has been larger owing to wavelength multiplexing. In current trunk-line optical communication systems, wavelength multiplexing communication is used and a wavelength channel spacing is determined. Accordingly, with 50 GHz spacing in the bandwidth of an optical fiber amplifier, about 100 channels can be used.
When the channel spacing is denoted by Δf[Hz] and the transmission rate is denoted by B[bit/s], B/Δf[bit/s/Hz] denotes spectral efficiency. Assume that Δf=50 GHz. In this case, with the transmission rate of 100 Gbit/s for each channel, the spectral efficiency is 2 bit/s/Hz.
Since the bandwidth of the optical fiber amplifier is limited, it is required to improve the spectral efficiency. However, a simple increase in the bit-rate of signals in order to increase the spectral efficiency causes a problem of crosstalk between channels. To address with this problem, optical multilevel modulation and optical orthogonal frequency-division multiplexing (OFDM) have been studied as next-generation optical communication systems. The optical multilevel modulation is different from the conventional optical intensity modulation that uses two values of 0 and 1, and is a method of increasing an amount of information without increasing the frequency usage bandwidth by performing multi-valuing using the amplitude and the phase of light. Meanwhile, in the optical OFDM, an OFDM signal is generated by an electric signal and the OFDM signal is optically modulated, and optical sub-carriers are multiplexed in a state in which the optical sub-carriers are orthogonally arranged. Accordingly, it is possible to solve the problem of crosstalk and to improve the spectral efficiency.
The transmitted optical signal after being subjected to multi-valuing or multiplexing mainly by electric signal processing as described above is demodulated into an electric signal on a receiving side. An analog/digital (A/D) converter needs to be provided at a subsequent stage of a PD (photodetector for light receiver) of an optical demodulator circuit. Recently, an A/D converter using an electric circuit has typically been used.
Meanwhile, many proposals have been made for an optical A/D converter that directly derives an analog amount of an optical signal as a digital value since it can be operated at high speed. For example, in PTL 1, a light signal is divided by predetermined division ratios different from one another to indicate the light amount by a predetermined ratio, thereby detecting an optical analog amount of an optical signal that is input depending on whether each of the divided optical signals reaches a threshold.
Further, in PTL 2, in optical A/D conversion means, a feedback system is formed through a non-linear optical element for an input light signal which is an analog signal, thereby sequentially obtaining first output light which is a digital signal from the optical A/D conversion means.
In PTL 3, an optical encoding circuit optically encodes a pulse train of signal light having a first wavelength according to control light which has a neighboring second wavelength different from the first wavelength and has a pulse train of an optically sampled optical analog signal, by using a plurality of optical encoders each including optical nonlinear devices having input-to-output characteristic with different periodicities with respect to the light intensity, and outputs a plurality of pulse trains of optically-encoded signal light from the respective optical encoders. Next, an optical quantization circuit performs optical threshold processing on each of the pulse trains of carrier wave light having a neighboring third wavelength different from the first wavelength according to the plurality of pulse trains of optically-encoded signal light to optically quantize the pulse trains of carrier wave light, by using a plurality of optical threshold processors each of which is connected to each of the optical encoders and includes a nonlinear optical device having a periodic input-to-output characteristic with respect to light intensity, and outputs optically quantized pulse trains as optical digital signals.
PTL 4 has characteristics in that a plurality of interferomatic optical modulators are provided, a photodetector device is formed on the same substrate, and an output voltage of the photodetector device is applied to the interferomatic optical modulators. Accordingly, in this example, since intensity signal light is converted into a voltage signal after being received by a PD once, the rate of the electric signal determines the rate of the whole circuit.
A phase difference can be used as a signal in addition to an intensity signal for light, and some PTLs include an apparatus using a phase difference as a signal. There are further examples that use this method to generate a phase difference for use from intensity without changing light into an electric modulation signal.
PTL 5 discloses a logical hold/logical inversion signal light generator 116 for converting an optical signal that is ON or OFF into a signal of phase difference using the optical signal.
PTL 6 discloses an apparatus that removes control light by a filter and uses a modulation signal of phase difference of light.